Graphite is a crystalline form of carbon. The carbon atoms in graphite form a layered structure wherein each carbon atom is strongly bonded to three other carbon atoms in the same layer by covalent bonds called sigma bonds, and is relatively weakly bonded to carbon atoms in adjacent layers by delocalized bonds called pi bonds. Accordingly, the layers of carbon atoms in graphite are weakly bonded to each other by a delocalized distribution of pi electrons, which may account for the lubricating and electrical conducting properties of graphite.
Because of the weak bonding between carbon layers in graphite, a variety of compounds that either accept or donate electrons can be interspaced between adjacent carbon layers in graphite to expand the graphite lattice. The compound interspaced between the carbon layers of graphite is termed, here and hereinafter, an "intercalant". During an interaction with graphite, the intercalant diffuses between adjacent carbon layers in graphite, and an electron exchange occurs between the intercalant and the carbon atoms of the graphite. Compounds resulting from the interaction between graphite and an intercalant are termed, here and hereinafter, "graphite intercalation compounds".
Depending upon the relative amounts of graphite and the particular intercalant that are interacted, different graphite intercalation compounds result. These different compounds are termed "stages" of a particular graphite intercalation compound. For example, a first stage graphite intercalation compound has alternate layers of carbon and the intercalant; a second stage graphite intercalation compound has two successive carbon layers of graphite followed by a layer of intercalant; a third stage graphite intercalation compound has three successive carbon layers of graphite followed by a layer of intercalant; and so on.
Nonintercalated graphite is known and used as a lubricant and as an electrical conductor. An intercalant improves the mechanical properties of graphite, and especially the self-lubricating properties of graphite. However, conventional methods of manufacturing graphite intercalation compounds have the following disadvantages: 1) production of a nonuniform product that includes excess, i.e., non-intercalated, intercalant, and 2) low production volumes. A need exists to overcome these disadvantages.
One conventional method of manufacturing a graphite intercalation compound utilizes a static bed reactor. In this method, the graphite and the intercalant, such as a metal halide, first are admixed, then the mixture is introduced into a resealable static, i.e., fixed, bed reactor. After sealing the reactor, the mixture is heated. However, a simple mixture of graphite and an intercalant, in the absence of an interactive gas, does not interact, even at an elevated temperature, to form a graphite intercalation compound. An interactive gas, such as chlorine gas (Cl.sub.2), therefore is passed through the heated mixture of graphite and intercalant to form the graphite intercalation compound. The interactive gas can be generated in situ or can be passed through the graphite-intercalant mixture as a gas stream. In general, the interactive gas mediates electron transfers that allow the intercalant to become interspaced between the adjacent carbon layers of graphite.
A conventional fixed bed reactor includes an inlet for the interactive gas stream and an outlet for interactive gas stream and other interaction byproducts. Therefore, a stream of an interactive gas is introduced to the fixed bed reactor and allowed to pass through the heated graphite-intercalant mixture to provide a graphite intercalation compound.
The above-described conventional method of manufacturing a graphite intercalation compound yields only about 3.5 pounds of graphite intercalation compound per 8 hours. Design limitations for static bed reactors essentially preclude the assembly of large scale reactors, thereby limiting the static bed reactors to pilot plant scales and, accordingly, low volume yields.
Furthermore, a graphite intercalation compound resulting from the conventional static bed method is a nonuniform product because: 1) packing of the graphite-intercalant mixture in the fixed bed reactor prevents the stream of interactive gas from passing through the graphite-intercalant mixture uniformly across the cross-section of the fixed bed reactor; and 2) a heat gradient exists over the crosssection of the fixed bed, wherein the center of the fixed bed is cooler than regions of the fixed bed closer to the walls of the reactor. The packing of the mixture also results in channeling of the interactive gas through the fixed bed of graphite-intercalant mixture. Accordingly, there is non-uniform contact between the interactive gas and the graphite-intercalant mixture, further resulting in a non-homogeneous product.
After the interaction between the graphite and the intercalant is complete, the fixed bed reactor is cooled, then the top of the reactor is unsealed, and the graphite intercalation compound is removed from the bed of the reactor. The resulting graphite intercalation compound includes excess, i.e., non-intercalated, intercalant, and includes free, i.e., non-interacted, graphite. Usually therefore, a graphite intercalation compound prepared by a present-day commercial fixed bed method includes up to about 40% by weight excess intercalant, providing a nonuniform graphite intercalation compound that has decreased self-lubricating properties compared to a graphite intercalation compound that is essentially free of excess intercalant. In contrast, the present method provides a graphite intercalation compound that is essentially free of excess intercalant. In addition, the present invention allows the purification of a graphite intercalation that includes a large amount of excess intercalant, such as a graphite intercalation compound prepared in a fixed bed reactor, to provide a graphite-intercalation compound including 1% or less by weight excess intercalant.
Another method of manufacturing a graphite intercalation compound is disclosed in Knappwost, U.S. Pat. No. 3,377,280, wherein graphite and a metal powder are heated in the presence of a halogenating agent, like chlorine gas, to provide a graphite-metal halide intercalation compound. The Knappwost method purportedly overcomes the difficult and commercially unattractive expense of heating graphite and a metal halide to form a graphite intercalation compound.
Oblas et al., in U.S. Pat. No. 4,604,276, and Su, in U.S. Pat. No. 4,608,192, disclose the preparation of a graphite intercalation compound in a vapor-transport reactor. The method of Oblas et al. utilizes metal halide vapors generated in a first zone of a reactor to intercalate graphite in a second zone of the reactor. The resulting graphite intercalation compound is maintained at a temperature sufficiently high to prevent condensation of the metal halide on the graphite intercalation compound, and thereby a more pure and uniform product is provided. Su discloses a similar method.
Sugiura et al. U.S. Pat. No. 4,729,884 discloses the preparation of a graphite metal chloride intercalation compound without the use of chlorine gas. The method of Sugiura et al. utilizes both (1) a first metal chloride to intercalate the graphite, and (2) a relatively small amount of a second metal chloride, having a lower boiling point than the first metal chloride, as a substitute for the chlorine gas. Although the method of Sugiura et al. increases the reaction rate for producing a graphite intercalation compound, the intercalation compound can also include some second metal halide intercalated into the graphite, which is undesirable because the presence of two metal halides can adversely affect the properties of the desired graphite intercalation compound.
It would be desirable to provide a simple and economical method of manufacturing a graphite intercalation compound in high volume, and in a relatively short reaction time. It would also be desirable to provide a graphite intercalation compound which is uniform and is essentially free of excess intercalant, and to provide a method of purifying a graphite intercalation compound by removing excess intercalant.